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From: "Ashlock,William" <William.Ashlock@edwards.boc.com>
To: rsgb_lf_group@blacksheep.org
Date: Mon, 25 Nov 2002 14:26:50 -0500
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Subject: LF: RE: loop antenna
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Reply-To: rsgb_lf_group@blacksheep.org

Hi Rik,

>I am about to put up a transmitting loop antenna. Dimensions will be 23m 
>long and 15m high. Thanks to Dick (PA0SE) I could achieve a wheel of 7mm 
>litz wire, so wire losses should be minimized (about 0.04 Ohm).

Delighted to read about what you are up to!

First. Is 7mm the actual wire diam or the wire plus insulation?

>What distance should I keep to the ground ? 

I like 6 feet as the best compromise. The ground loss will amount to about
1/2 of your total loss loss with a loop this size and conductor Rac this
low. I'll give you a number later, based on my data. 

>What distance should I keep to the greenery (birch trees)

The trees will have almost no effect. I run my conductor (now a 1/2" Cu
pipe, in full contact with the trees (see my loop website at
http://www.ThePittsfieldInn.com/Bill/ for a picture of this), but it's a
good idea to have some spacing at the lower portions of the loop where the
voltage to ground is the highest. Also, you will have to protect the Litz
wire from abrasion from the trees. That's a valuable piece of cable!  
 
>How to match the antenna ? As it is an experimental setup I would tend to 
>keep the resonance tuning (= series capacitor) and impedance matching (= 
>transformer) separated.

DO NOT separate these components! This a very common mistake. The
transformer and capacitor bank must be next to the loop feed point or the
losses will kill you.

>Any experience with a 100/1 .. 400/1 transformer 
>(matching 50 Ohm of the TX to  0.1 .. 0.4 Ohm of the loop - at least I hope

>to get losses that low).

I have no experience with high power cores but the ratios look correct.
Suggest a focussing some attention to core losses.

>if Rloss = 0.2 Ohm and 450W RF power -> antenna current  = 47A  -> EIRP 
= 675mW

A loop this large will perform well, Rik, if you pay close attention to the
transformer, and capacitor bank. The effectiveness once the area is fixed is
all in the loop current.

Good luck!

Bill A


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